The prokaryotic transcript cleavage factor GreA and GreB are presumed to have two biologically important and evolutionarily conserved functions: the suppression of transcription arrest and the enhancement of transcription fidelity. These functions are accomplished by the ability of Gre factors to induce the cleavage of the nascent RNA in ternary complexes of RNA polymerase. The broad goal of this project is to understand the molecular mechanism of action and the structure-functional relationships of GreA and GreB in Escherichia coli. For this purpose, three types of experiments will be conducted. First, to identify functionally important localities of Gre factors, the amino acid residues of Gre A and Gre B that, according to their established 3-D structure, are located on the protein surface will be mutagenized. The mutant proteins will be then characterized biochemically using specific in vitro transcription assays and structurally by X-ray analyses. The second type of experiments are aimed at detailed studies of the basic "patches" of Gre molecules formed by positively charged surface-exposed residues. To this end, a series of Gre mutants will be constructed that carry basic patches of various sizes and the resulting mutant factors will be analyzed functionally an biochemically. A model that implicates the basic residues of the patches in activation of the intrinsic nucleolytic site in RNA polymerase will be tested. Finally, the interactions of Gre A and Gre B with RNA polymerase will be studied using protein-protein photochemical crosslinking. The Cys residues will be introduced into Gre proteins by site-directed mutagenesis of selected surface-exposed residues followed by their derivatization with thiol-specific photoactive bifunctional reagents. The resulting modified Gre proteins will be used to probe the interactions with RNA polymerase at different stages of transcription elongation.